The present invention relates to a process in which solutions containing nitrate(s) and/or other nitrogen-containing compounds (contaminated solutions) are treated with SO2 gas in the presence of an at least equimolar amount of NH4+ or an ammonium salt.
Nitrates such as ammonium nitrate are fertilizers which are used in agriculture. Nitrates are also formed in chemical processes. Nitrates, in some cases, pass into rivers where they cause over-fertilization. For that reason, the introduction of nitrate-containing wastewater is prohibited. Nitrates that form in chemical plants in various processes, for example in nitrations or in the preparation of hydroxylamines according to Raschig, are usually converted into nitrogen by denitrification in biological purification plants by an appropriate procedure. For the reduction of the nitrate oxygen, however, it is necessary to add organic substances. The consequence of the biological activity is, however, a growth in the activated sludge, which must be removed with thickeners, centrifuges, flocculating agents and driers. The destruction of nitrate in that manner is not only expensive but also frequently difficult to manage, because small amounts of toxins can destroy biological activity. In addition, relatively small variations in the amount of nitrate are problematical for complete conversion.
Other methods for removing nitrates are, for example, distillation and extraction (Ullmann""s Encyclopedia of Industrial Chemistry (1991) A 17 page 324-325; EP-A 0 396 790). In that case, although it is possible to remove the nitrate from a particular production stream, contaminated by-products form as an extraction residue (in the case of extraction) or as a distillation residue (in the case of distillation) and must in turn be treated in a complex operation.
It is an object of the present invention to provide an alternative, technically simple process that allows nitrates and also organic and inorganic nitrogen compounds to be removed as completely as possible, preferably totally.
This and other objects which will be apparent to those skilled in the art is achieved by a process that converts nitrates to nitrogen by chemical means, specifically, by treatment of the water with SO2 gas in the presence of sulfuric acid and an at least equimolar amount of ammonium or an ammonium salt at a temperature above 65xc2x0 C. but below boiling. For example, in the case of caprolactam plants which use hydroxylamine according to the Raschig process as the raw material, treatment in accordance with the process of the present invention prevents formation of nitrate-containing waste water because the ammonium sulfate solution which remains can be processed to produce fertilizers.